1. Field of the Invention
The present invention relates to an active matrix substrate used in a liquid crystal display device or the like, and a method for fabricating the same.
2. Description of the Related Art
In recent years, the development of a thin-film transistor (hereinafter, referred to as a "TFT") has been conducted in order to apply the TFT to image display devices such as flat panel displays including a liquid crystal display device. In particular, the development of a driver monolithic type liquid crystal display panel in which polycrystalline silicon TFTs are used so as to form a display unit and a driving circuit unit on the same substrate has been vigorously conducted.
FIG. 3 is a plan view illustrating such a driver monolithic type active matrix substrate. In FIG. 3, the reference numeral 20 denotes a display unit. The reference numerals 21 and 22 denote a driving circuit unit for data signals and a driving circuit unit for scanning signals, respectively. Each of the driving circuit units 21 and 22 is provided in the periphery of the display unit 20. Data signal lines 23 and scanning signal lines 24 are connected to the driving circuit unit 21 for data signals and the driving circuit unit 22 for scanning signals, respectively.
As shown in FIG. 4, a pixel TFT 25 as an active element and a pixel electrode 26 are connected to the vicinity of the intersection between the data signal line 23 and the scanning signal line 24 in the display unit 20. A scanning signal from the driving circuit unit 22 for scanning signals drives the pixel TFT 25, and a data signal voltage from the driving circuit unit 21 for data signals is applied to the pixel electrode 26.
FIG. 5 is a cross-sectional view illustrating the structure of a TFT in this active matrix substrate. In FIG. 5, the reference numeral 27 denotes a transparent insulating substrate. The reference numeral 28 denotes a semiconductor layer having a channel region 29 and low resistance regions 30. The reference numerals 31 and 32 represent a gate insulating film and a gate electrode, respectively. The reference numerals 33 and 34 represent an interlayer insulating film and source and drain electrodes, respectively. The active matrix substrate includes stagger type TFTs in which the semiconductor layer 28 of the TFT is composed of polycrystalline silicon.
The above-described driving circuit unit 21 for data signals or the driving circuit unit 22 for scanning signals shown in FIG. 3 generally employs a clocked inverter shown in FIG. 6 as an element in an output unit provided therein or the like. The clocked inverter includes: N-channel type TFTs 35; P-channel type TFTs 36; clock signal lines 37 for driving the TFTs 35 and 36; and constant voltage lines 38 for supplying voltage. Such a complementary type clocked inverter where the N-channel type TFTs 35 and the P-channel type TFTs 36 are combined realizes a higher processing speed of the circuit and lower power consumption as compared to the case where the clocked inverter is composed of the N-channel type TFTs alone. FIG. 7 is a plan view illustrating a pattern of the clocked inverter shown in the left side of FIG. 6.
Hereinafter, a method for fabricating the conventional active matrix substrate will be described with reference to FIGS. 8A and 8B illustrating portions of the clocked inverter.
First, as shown in FIG. 8A, polycrystalline silicon thin films 39 are formed as semiconductor layers on a transparent insulating substrate. Then, a SiO.sub.2 film is formed so as to form a gate insulating film (not shown). Next, an Al alloy thin film is used to form gate electrodes 40 and a line 41 for a crossing portion in a pattern as shown in FIG. 8A.
Subsequently, as shown in FIG. 8B, an n-type low resistance region 42 and a p-type low resistance region 43 are formed in a pattern as shown by hatching using an ion doping method or the like. Then, a SiO.sub.2 film is formed so as to form an interlayer insulating film, and contact holes 44 are formed in the interlayer insulating film. Next, clock signal lines 37 and constant voltage lines 38 are formed by the patterning of the same type of thin metal film, i.e., a thin metal film for data signal lines. In this manner, the clocked inverter portion as shown in FIG. 7 is fabricated.
In such an active matrix substrate, defects in the driving circuit units 21 and 22 having the clocked inverter and the like, immediately leads to defects in the display unit 20. As a result, it is extremely important to improve yield of the driving circuit units 21 and 22. In the conventional driving circuit units shown in FIG. 7, however, due to a large number of the contact holes, these driving circuit units are susceptible to connection failure and the influence of static electricity. Moreover, due to their long line length and a large number of intersections between the lines, breakage of lines is more likely to occur.
As a result, the rate of connection failure at the contact holes and the number of line breakage increase, resulting in an unsatisfactory yield of the active matrix substrate as a product. In order to prevent such line breakage, Japanese Laid-open Publication No. 2-285678 suggests a technique for making a portion of the clock signal line and/or the constant voltage line a double line. However, since such a technique is more likely to be affected by static electricity, the rate of dielectric breakdown at the TFT is high. Thus, with such a technique, an increase in the yield has not been accomplished yet.
Accordingly, there is a need for an active matrix substrate having the reduced number of contact holes and excellent yield.